1. Field of the Invention
The present invention relates to a sheet decurling device and an image forming apparatus, and more particularly, to a structure that removes a curl of a sheet that occurs during a transporting process.
2. Description of the Related Art
In an image forming apparatus such as a multi function peripheral (MFP), a printer, a facsimile etc., visual images such as toner images, which are carried on a surface of photosensitive drums that are latent image carrying members, are transferred onto a sheet such as a recording sheet. The sheet is fixed by a fixing device using addition of heat and is ejected.
In the fixing device, a toner image carrying surface of the sheet touches a heating source member such as a heating roller and heat and pressure are used to cause the toner to melt and penetrate the sheet, thus fixing the toner on the sheet.
Upon receiving the heat from the heat source member, moisture of the sheet that is subjected to fixing evaporates, thus causing a change in a moisture content percentage of a sheet surface that is touching the heating source member and a back surface. Change in the moisture content percentage causes occurrence of warping in a thickness direction, in other words, a rolling tendency called a curl.
When the curled sheet is ejected and stacked, end edge positions of the stacked sheets do not align, thus necessitating an operation to align side end edges of the sheets when stacking is completed.
In one of the existing structures that is disclosed in Japanese Patent No. 3050633 and that is used to curb a curl, the sheet that has passed the fixing device is caused to touch a cooling member such as a heat pipe, thus curbing evaporation of the moisture content. Alternatively, in a structure that decurls the sheet and that is disclosed in Japanese Patent Application Laid-open No. H3-44985, among a pair of rollers that sandwich-transport the sheet, a speed of the roller opposite to a curled surface side of the sheet is enhanced at the time of sandwich-transportation of the sheet and the sheet is forcibly beaten to cause occurrence of curl such that the surface of the sheet opposite the roller with the enhanced speed is rolled around the side of the roller.